1. Field of the Invention
The present invention relates to a polishing machine, and more specifically to a polishing machine with an improved polishing pad structure, used for planarizing a concavo-convex surface of a wafer in a semiconductor device manufacturing process.
2. Description of Related Art
In a semiconductor device manufacturing field, recently, a chemical mechanical polishing (abbreviated to "CMP") has been used for polishing convex portions of a wafer surface in order to planarize the wafer surface.
In order to carry out this CMP, for example, a polishing machine as shown in FIG. 3 is used. This prior art polishing machine mainly comprises a rotating surface plate 3 having an upper surface on which a polishing pad composed of an upper polishing web 1 and a lower polishing web 2 are spread, a polishing liquid supply mechanism 6 for supplying a polishing liquid containing a polishing powder (abrasives) 7 onto the upper polishing web 1, and a wafer holder 4 for holding a wafer 5.
The rotating surface plate 3 has a center shaft 3S coupled with a motor 3M, so that the rotating surface plate 3 is driven to rotate in a direction "A". On the other hand, the wafer holder 4 has a center shaft 4S coupled with a motor 4M included in a not-shown driving mechanism, so that the wafer holder 4 is driven to rotate in a direction "B" and also to move in a direction "C". Here, a mechanism for moving the wafer holder 4 in the direction "C" is omitted in the drawing for simplification of the drawing, since the shown polishing machine is well known to persons skilled in the art. With this arrangement, the wafer 5 and the upper polishing web 1 can be brought into a sliding contact with each other, and also can be separated from each other.
In order to actually polish the wafer by use of the above mentioned polishing machine, first, the wafer 5 is held by the wafer holder 4 in close contact with the wafer holder 4, and is rotated together with the wafer holder 4. On the other hand, the rotating surface plate 3 is rotated, and the polishing liquid containing the polishing powder 7 is supplied onto the polishing web 1 from the polishing liquid supplying mechanism 6. In this condition, the wafer 5 is brought into a sliding contact with the upper polishing web 1 with the polishing powder 7 being interposed between the wafer 5 and the upper polishing web 1. As a result, the wafer 5 is polished. Here, the upper polishing web is formed of a hard material, so that a concavo-convex surface of the wafer 5 is planarized. On the other hand, the lower polishing web is formed of a soft material, for the purpose of making possible to polish the wafer while following a contours of the wafer.
Specifically, in order to obtain a good polished planarization without damaging the wafer 5, a polishing sheet formed of a foamed polyurethane having the hardness controlled to a predetermined hardness is used as the upper polishing web 1. On the other hand, in order to follow the contours of the wafer, a non-woven fabric formed of polyurethane fibers is used as the lower polishing web 2.
When the non-woven fabric is used as the lower polishing web 2 as mentioned above, a water content of the polishing liquid containing the polishing powder 7 inevitably immerses into the lower polishing web 2 through an exposed peripheral end 2E of the lower polishing web 2, with the result that the hardness of the lower polishing web 2 lowers, and as shown in FIG. 4, the polishing rate of a peripheral portion of the wafer 5 becomes larger than that of a center portion of the wafer 5. Why this phenomenon occurs will be described with reference to FIG. 5.
Referring to FIG. 5, because of the rotation of the rotating surface plate 3, the upper polishing web 1 and the lower polishing web 2 move in relation to the wafer 5 in a direction "D" which corresponds to a circumferential direction of the rotating surface plate 3. As a result, as shown in FIG. 5, a peripheral portion 5E of the wafer 5 firstly contacting with the moving upper polishing web 1 dents into the upper polishing web 1 because of friction with the upper polishing web 1. At this time, if the lower polishing web 2 contains the water, since the hardness of the lower polishing web 2 has become low, the denting amount becomes large. As a result, the wafer 5 is inclined on the upper polishing web 1, so that the load is concentrated onto the peripheral portion 5E of the wafer, and therefore, the polishing rate becomes large at the peripheral portion 5E of the wafer.
In order to overcome this problem, Japanese Patent Application Pre-examination Publication No. JP-A-08-241878 (an English abstract of JP-A-08-241878 is available from the Japanese Patent Office and the content of the English abstract of JP-A-08-241878 is also incorporated by reference in its entirety into this application) proposes a polishing pad as shown in FIG. 6. This proposed polishing pad is characterized in that an lower polishing web 2A has a number of square pillars separated from one another, and an upper polishing web piece 1A having an area larger than a top area of the square pillar is adhered onto the top area of the square pillar, so that an eaves of the upper polishing web piece 1A is formed on the square pillars of the lower polishing web 2A. This structure is intended to uniformly polish the wafer.
However, since a gap exists between the upper polishing web piece 1A, the water inevitably immerses into the lower polishing web 2A, so that the hardness of the lower polishing web 2A changes, and therefore, the polishing characteristics inevitably changes.